The invention relates to a circuit and method for accurately measuring the temperature of a "hot spot" of a "functional circuit" in an integrated circuit semiconductor chip without diminishing the accuracy of the measurement and to accurately and predictably shut down at least a portion of the functional circuit when a predetermined maximum acceptable hot spot temperature is exceeded.
Excessive "hot spot" temperatures due to high power dissipation of a functional circuit in an integrated circuit chip can result in permanent circuit damage if the resulting excessive hot spot temperature is not promptly detected and reduced by turning off or powering down at least part of the functional circuit causing the power dissipation. Such hot spot temperatures usually are difficult to measure because they occur at locations where power dissipation is highest, and if additional sensing circuitry also is located in the high dissipation area to sense the resulting hot spot temperature, the very act of locating the sensing circuit reduces power density near the hot spot area and therefore tends to "corrupt" the measurement. Consequently it is difficult to provide a "thermal shutdown" which occurs at a consistent and predictable hot spot shutdown temperature. In the prior art, temperature sensing circuitry has been placed as close as possible to the "hot spot" area to achieve the most accurate hot spot temperature sensing capability. Another known technique is to deliberately set the threshold temperature at which a shutdown circuit is activated to a value lower than the actual desired maximum hot spot temperature. However, that approach is likely to result in "overdesign" of the integrated circuit chip, by making it larger and hence more costly than otherwise would be necessary to reduce power density, or in overdesign of the heat sinking structure, again resulting in higher cost.
U.S. Pat. No. 5,563,760 (Lowis, et al.) and U.S. Pat. No. 5,444,219 (Kelly) disclose temperature sensing circuits that include a first pair of resistors located close to an area in which the temperature is to be sensed, and a second pair of resistors located further from the area in which the temperature is to be sensed. The problem with this prior art approach, and also with prior art circuits in which PTAT (proportional to absolute temperature) circuits are located within a high power dissipation region, is that all of these prior art circuits reduce the power density in the hot spot region and thereby diminish the accuracy of measurement of the hot spot temperature and therefore do not result in a reliable, optimal thermal shutdown of the circuitry dissipating excessive power.
So called PTAT (proportional to absolute temperature) circuits which produce an output current that is proportional to the absolute temperature of the transistors constituting the circuit are well known. Ordinarily, the four transistors of a PTAT circuit are deliberately located as close together as possible so that the PTAT output current represents the absolute temperature of the silicon region in which those transistors are located, and no temperature gradients come into play in the design of prior PTAT circuits.
Thus, there is a presently unmet need for an improved circuit and technique for increasing the reliability and lifetime of integrated circuits having highly localized "hot spot" areas.